1. Field of the Invention
The present invention relates to an electronic apparatus, which uses a display device such as a TFT (Thin Film Transistor) active matrix type liquid crystal display (to be referred to as a TFT LCD hereinafter) for holding display data in units of pixels.
2. Description of the Related Art
In recent years, electronic apparatuses such as personal computers have been rendered compact and lightweight, so that they can be easily carried and used anywhere. Furthermore, in order to allow a long-time use of the electronic apparatus using a battery power supply, compact, lightweight batteries having a large electric capacity, and power saving mechanisms of the electronic apparatuses have been developed.
An automatic sleep mode is an example of a power saving mechanism. In this mode, when no keyboard input is detected for a predetermined period of time, a system is automatically set in a sleep state. Thereafter, when a keyboard input is detected, the system is returned to a normal operation state.
In a screen display operation of a conventional electronic apparatus, which comprises a display device such as a CRT (cathode ray tube), a plasma display, an STN type LCD, or the like, a read operation of display data from a VRAM (video RAM), an output operation of display data to the display device, and a screen display on the display device are periodically performed independently of the presence/absence of a change in display content on the screen. This is because, in this display device, the screen display disappears when display data are not periodically supplied. Also, in an electronic apparatus, which comprises a display device such as a TFT LCD having a function of storing display data in units of pixels, a read operation of display data from a VRAM, an output operation of display data to the display device, and a screen display on the display device are periodically performed.
Since the TFT LCD originally has a function of storing display data in units of pixels, when the same screen display content continues, the read operation of display data from the VRAM is actually unnecessary. A state wherein the same screen display content continues occurs very frequently. For example, when a wordprocessor software program is used in a personal computer, an operator does not often change a screen display content when he or she thinks of a composition in front of the display screen or when the computer is executing complicated computation processing. Such a state also occurs when data on a window, which is not displayed on the screen, is rewritten in an operation using a window function, or when the same data is input at the same position. However, in a conventional system, a VRAM, a display controller, and the display device are always operated to perform a screen display, and power consumption of the screen display operation cannot be reduced.